1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and methods for use thereof.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronic circuits. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays, More information on mirror arrays as here referred to can be gleaned, for example, from United States Patents Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as here above set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed 27 Feb., 1998 (WO 98/40791), incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0672504, incorporated herein by reference.
Alignment is the process of positioning the image of a specific point on the mask to a specific point on the wafer that is to be exposed. Typically one or more alignment marks, such as a small pattern, are provided on each of the substrate and the mask. A device may consist of many layers that are built up by successive exposures with intermediate processing steps. Before each exposure, alignment is performed to minimize any positional error between the new exposure and the previous ones, such error being termed overlay error.
However, some of the intermediate processing steps, such as chemical mechanical polishing (CMP), rapid thermal annealing, thick layer deposition and deep trench etching, can damage or distort the alignment marks on the substrate or bury them under opaque layers. This can cause overlay errors.
In some technologies, such as micro systems technology (MST) and micro electromechanical systems (MEMS), devices are fabricated from both sides of a substrate. There has been a problem with performing exposures on one side of a substrate such that they are accurately aligned with features previously exposed on the other side of the substrate. An alignment accuracy of the order of 0.5 microns or better is typically required.
New techniques are developed that allow the imaging of patterning devices, having even smaller patterns, by using so-called immersion techniques. This technique is based on the fact that the space between the lens and the substrate contains a fluid, such as water. Since the refractive index of water is higher than the refractive index of air of some low pressure gas, the numerical aperture of the system is increased. This allows to image even smaller patterns, while using the same radiation system and projection system. More information about immersion techniques can for instance be found in EP 1 420 298 A2, EP 1 429 188 and EP 1 420 300 A2.
The presence of water will however influence the alignment measurements. As a result of the presence of water in the space between the lens and the substrate, it becomes difficult to perform alignment measurements on marks provided on the substrate. Furthermore, comparing such ‘wet’ measurements with previously obtained ‘dry’ alignment measurements is also rather difficult.
In case a so-called dual stage lithographic apparatus is used, in which alignment measurements may be performed in a first ‘dry’ measurement position and exposure is done on a second ‘wet’ exposure position, still measurements must be performed at the second ‘wet’ position. When the substrate stage is ‘swapped’ from the ‘dry’ measurement position to the exposure position, the substrate stage also changes from first to second alignment devices, the first alignment devices being used at the first ‘dry’ measurement position and the second alignment devices being used at the second ‘wet’ position. Therefore, additional measurements must be performed at the exposure position, i.e. the ‘wet’ position, in order to use the previously obtained ‘dry’ measurements during exposure, thus at the exposure position. Also, the actual alignment of the substrate relative to the reticle can only be performed at the exposure position, i.e. the ‘wet’ position, since no reticle is present at the measurement position.
In case a single stage lithographic apparatus is used, all measurements must be performed at the exposure position.
In both cases, single and dual stage, alignment measurements must be performed at the ‘wet’ exposure position, while these measurements are preferably obtained in a ‘dry’ environment. A straightforward solution to align substrates on the ‘wet’ exposure position would be to perform all alignment measurements before the water is added. However, this would significantly reduce the throughput of the system.
The possibility to perform alignment measurements during applying water is not possible according to the prior art. This will briefly be explained. In general, two alignment arrangements may be used: an off-axis alignment arrangement and a so-called through-the-lens alignment arrangement (TTL).
An off-axis alignment arrangement can not be used during applying water in the space in between the lens and the substrate, since the off-axis alignment arrangement must, in order to perform measurements, come close to the surface of the substrate. In order to apply water, the lens must also be brought in the vicinity of the substrate. Because of the dimensions of the lens as well as the off-axis alignment arrangement, it is not possible to have them both at the desired position at the same time, making it impossible to perform off-axis alignment measurements while applying water at the same time or while water is present.
Furthermore, it is not possible to perform a TTL-measurement during applying (filling) water. Such a TTL-measurement performs measurements by projecting an alignment beam via an alignment mark provided on the reticle, through the lens to an alignment mark provided on the substrate stage. Underneath this alignment mark provided on the substrate stage a measurement arrangement may be provided, such as for instance a light detector. It will be understood by a person skilled in the art that it is impossible to perform such measurements during applying water, since the rising water level would change the optical density and thus would change the path of the alignment beam and therefore disturb the measurements.
Techniques for performing TTL-alignment after applying water, thus when the water is present, are not developed yet. In some cases, such ‘wet’ measurements will need to be compared with previously obtained ‘dry’ measurements, which is also not straightforward. Finally, since applying water is a time-consuming action, the need arises to develop a more time-efficient procedure.